1. Field of the Invention
The present invention is related in general to the field of communications and specifically to frequency offset in a satellite network.
2. Discussion of the Background
Satellite communication systems are capable of providing telephone and broadband data communications over a large geographical area. A satellite may be used to form a network so that two or more remote terminals can communicate with each other by relaying the information over the satellite. Two common types of satellite networks are star networks and mesh networks.
FIG. 1 shows an exemplary form of a star satellite network. A star network usually has a central terminal, such as hub 110. In a star network, remote terminals, such as terminals 111 and 112, can communicate with the hub 110 through the satellite 100. In order for one remote terminal, to communicate with another remote terminal, the data must first pass through the hub 110, requiring two relays (or “hops”) through the satellite 100.
FIG. 2 shows an exemplary form of a mesh satellite network. In a mesh network, remote terminals, for example remote terminals 111 and 112, are capable of communicating directly with other remote terminals via the satellite 100.
In order for a communications receiver to demodulate the information from a received signal, it is often helpful for the receiver to have an accurate estimate of the signal frequency. When a receiver is tuned to a frequency that is not the same as the frequency of the received signal, a frequency offset error can exist. If this offset is too large, it may not be possible to reliably recover data from the signal. A manner of providing a more accurate frequency estimate may reduce the need for further frequency tracking or estimation by a receiver, potentially reducing the cost and complexity of the receiver. Additionally, a more accurate frequency estimate could allow for the performance of the receiver to be improved.
Satellite systems often have significant sources of frequency offset error that can degrade communications performance or increase the cost of a satellite network. Many components of the satellite network, such as upconversion equipment, downconversion equipment and the satellite transponder, can translate the frequency of a signal to accommodate various physical and regulatory requirements. When each component that translates the signal does not utilize same frequency reference source, each translation may produce a frequency offset that contributes to an overall frequency offset error between the receiver's tuned frequency and the actual received signal frequency. Frequency offset errors may also be caused by temperature fluctuations of electrical equipment or environmental conditions.
In star networks, the hub can be used to simplify network operation and lower cost of the network. The hub may assist each terminal in communicating with the hub by providing the terminals with network control and status information. This information may include a time reference, a frequency reference, measured frequency offset information, measured time offset information, and other information that can improve the performance and lower the cost of the terminals.
Mesh networks, however, which require only one satellite hop for remote to remote communication, may operate with only approximately half of the transmission delay found in remote to remote transmissions of star networks. Transmission delay can have an adverse effect on telephony and other applications using the satellite network. Additionally, mesh networks can reduce the cost of satellite access because the information only passes through the satellite once.
Mesh networks, however, may have terminals operating using different time and frequency reference sources. This may result in time offsets and frequency offsets that can degrade the performance of the network and make it difficult for remote terminals to communicate with one another.